I use both PIC and Zilog products (actually using the eZ80 in a couple of things, haven't used the Z8 itself in years) and I'd have to say I much prefer the Zilog.
There's nothing really all that "wrong" with the PICs in my opinion, I just don't actually "like" them very much.
A primary consideration these days has to be the cost of development tools and the ease of bringing a product to prototype quickly. Tools available free of charge or at a very low cost assist this. Also products that provide in circuit programming and background debug are in my view essential (spent years fiddling with hugely expensive ICE and am glad I don't have to any more).
We're using flash based PICs in a couple of products that require very low cost and simple functionality. These prodcts have very specific functionality so I can write the code, test it thoroughly, then never need to go near it again (hopefully!!). PICs are great for this kind of thing. Whenever I have to develop one of these I put my PIC head on, eventually get my fingers to type the correct syntax, eventually remember which registers are in which block and quite rapidly get the code to work with the help of the simulator.
Anything that looks like the software could develop over time or where I believe I'm going to be into a bit of code re-use, or even when I think the code could get complicated, I move over to Zilog. The tools are pretty reasonable (now I'm used to where the bugs are), they didn't cost much either and I don't have any of the limitations I have with the PIC. I know I've got the performance to get me out of a hole if I find myself having to do some heavy processing, I know I'm not going to run out of code space or RAM too quickly and the background debug means I can test in real time on the real hardware.
I comment like crazy and write extensive design documents for every piece of software I write, not because we have company procedures that require it (which we do, written by me and enforced on the other guys who write code) but mostly because I have a terrible memory and find it really hard to remember why I did things a particular way. Even so, I still find it hard to pick up the PIC and just start writing code without looking at some code first to remind myself of the syntax.
I've been writing code on zilog products for over 20 years. I don't see me stopping any time soon.